The clear trend in the computer industry today is toward the further development of portable computer systems. For example, notebook computers, laptop computers, and personal data assistants (PDA) are becoming increasingly popular and have already enjoyed widespread success in the marketplace. In many cases, these computer systems are designed to operate utilizing a power supply generated by a battery. Storage devices in laptop and notebook computer systems, such as disk drives, consume a significant amount of power when in operation. For example, a 2.5″ disk drive may consume up to 20% of total system power while a 1.8″ drive may consume up to 15%. Because of the continuous power drain associated with battery-powered systems there has been an emphasis on developing circuits and systems which are either capable of operating storage devices at reduced power levels, or which feature a low-power mode of operation of the storage devices.
The primary power management technique for power management in using disk drives in computers is the use of several reduced-power or power-save operating modes, each mode being entered following time out of a fixed predetermined time period since a disk drive read or write operation. For example, at the end of a fixed time period since the user last wrote data to the disk or read data from the disk, the read/write heads are moved to their parking location and the disk drive spindle motor is shut off. When the user next accesses the disk drive, the spindle motor is spun up and the heads are moved across the disk to read or write data on the appropriate data track.
The primary disadvantage of such a power-save mode is the time delay in exiting the mode, during which the user must wait. Such delay greatly affects the performance of the computer. Typically, the lengths of the fixed time periods are set by the computer user through software. However, fixed times for entering power-save modes are a poor tradeoff between energy and performance since there is no responsiveness to the user workload. The user must change the fixed times in anticipation of the workload, and the selection of times too short or too long can adversely impact performance and energy consumption. Moreover, this approach (typically performed in software external to the disk controller) suffers from a number of disadvantages, such as: a) there is a considerable time delay for a disk drive in the off state to come up to speed (typically on the order of a few seconds—a delay that may in some circumstances be unacceptable to many users); b) there is considerable power required to bring a disk drive from an off state up to speed (which in some circumstances, may offset the benefits of de-powering it); and c) frequent de-powering/re-powering of a drive increases its likelihood of failure. Thus, it is apparent that fixed times for entering power-save modes are a poor tradeoff between energy and performance since there is no responsiveness to the user workload or the condition of power source.